Adults Bone Up On Calcium Needs

March 08, 1985|By Jane E. Brody. New York Times News Service.

NEW YORK — Calcium has become the nutrient of the `80s, eclipsing even salt as a cause for dietary concern. Though nearly every American grew up knowing that calcium builds strong bones and most of us were repeatedly admonished to drink lots of milk, which is calcium-rich, few realized that this mineral remained crucial to maintaining a healthy skeleton after they stopped growing.

The dairy industry`s slogan that ``you never outgrow your need for milk`` was dismissed by most as a sales pitch rather than sound dietary advice. But now, with a national epidemic of osteoporosis--weak bones that result in debilitating fractures--afflicting older Americans, especially white women over 60, tremendous attention is being focused belatedly on that bone-building mineral, calcium, and the foods and pills that supply it.

Unfortunately, much of the concern and the action taken to alleviate it seem misplaced. Many women are dosing themselves with costly, questionably effective or even dangerous calcium supplements. Even when the supplements taken are safe and effective in increasing the body`s calcium supply, there is little, if any, evidence that they will actually prevent the dreaded fractures and loss of height that result when bones deteriorate.

Furthermore, dietary calcium is but one element--albeit an important one

--involved in keeping bones strong. Other crucial factors that promote a loss of calcium from the body include excessive protein and salt intake, cigarette smoking and heavy caffeine and alcohol consumption. Many people have inadequate supplies of vitamin D, especially the activated form of this vitamin, which is needed for dietary calcium to be absorbed into the body. Finally, lack of physical exercise and the reduction of estrogen hormones after menopause lead to an accelerated loss of calcium from bones.

To appreciate the importance of these various factors, it helps to understand how bones are built and maintained. Though they appear to be solid and permanent, bones are actually ``fluid`` tissues. Their constituents constantly flow in and out as they are renewed and remodeled. Every five years the skeleton is completely renewed. If calcium is in short supply when this remodeling takes place, not enough of this mineral will enter the bones and they will gradually thin out and become porous.

Calcium is also lost from the bones when it is actively removed to supply other tissues with the calcium they need to perform life-sustaining functions. Evolution designed the human body to maintain a constant level of calcium in the blood (the level that is found in sea water). The bony skeleton is not just a framework for the body; it is also a storehouse of calcium that can be drawn upon like a bank account when the mineral is needed for some vital function, such as nerve transmission. When blood levels of calcium drop and the diet fails to fill in the gap, calcium is drawn from the bones. Repeated withdrawals without compensating deposits of calcium result in a depletion of the bone account and eventually osteoporosis (literally porous bones).

The ebb and flow of calcium in the blood is monitored by a hormone from the parathyroid gland. This hormone tells the body when more calcium is needed, triggering into action vitamin D, which is required for calcium to be absorbed from the intestines.

Many older Americans are deficient in vitamin D because they consume few foods that contain it and they are rarely, if ever, out in the sun, which triggers production of vitamin D in the skin.

The vitamin D that you consume in foods (such as fortified milk, liver, tuna and salmon) and that is made in your skin is an inactive form. To affect calcium absorption, it must first be changed into the active vitamin D hormone by the liver and kidneys. Dr. Hector DeLuca, a University of Wisconsin biochemist who first demonstrated this mechanism, showed that as people get older, their ability to activate vitamin D declines. Without enough of this hormone, lots of calcium in the diet is of limited value because the body will not be able to absorb it. And simply taking more vitamin D won`t help unless the body is able to convert it into the active hormone.

Experimental treatments with tiny amounts of vitamin D hormone in conjunction with calcium supplements have increased the density of bone and reduced the incidence of fractures. Treatments with calcium supplements alone have been shown to increase bone density in some, but not all, people, but their effect on the risk of fractures is not yet established. Treatment with estrogen hormones after menopause has been shown to reduce the risk of fractures, but the treatment may increase the risk of cancer in some women.